Tiit Pärnik is plant physiologist specialized in studies of the regulation of CO2 assimilation and relationships between photosynthesis and respiration in higher plants. T. Pärnik designed an apparatus and worked out original radiogasometric methods for the investigation of photosynthetic carbon metabolism and determination of photorespiratory and respiratory decarboxylations of primary and stored photosynthates.

These methods have been successfully applied in studies of the regulation of photosynthetic carbon metabolism and CO2 exchange in leaves of different plant species (starch accumulating and starch-deficient species, plants with different productivity, species with C3, C4, and C3-C4 type of photosynthesis) and of genetically modified plants. T. Pärnik has been involved in all the experimental research carried out in the Laboratory of Photosynthesis of the Department of Plant Physiology of the Institute of Agricultural and Environmental Sciences of the Estonian University of Life Sciences. His particular interests are related with interactions of photorespiratory and respiratory processes in different plant species under stress conditions.